The present invention relates to apparatuses for determining a characteristic of an analyte by measuring light modulated by the analyte.
The desire to measure an increasingly complex array of materials in a broad spectrum of environments is ever increasing. For example, since the advent of radioimmunoassay in the 1950's, a large number of additional protocols have been developed for measuring a large number of physiologically active compounds. As biochemistry and molecular biology have rapidly advanced, numerous compounds have been added to an already extensive list. Toward this end, there have been increasing efforts to develop new techniques and new apparatuses for rapid easy determination of analytes of interest.
The disciplines involved in the development of assays and apparatuses for measuring the assay results are quite diverse. Chemistry, physics, immunology, enzymology, and electronics are only a few of the disciplines which have found application in the field of chemical assays. Therefore, it is not always apparent that devices which find application in one field can be modified to be useful in a different field.
For many applications, a light-modulating sample spot is produced on a surface, for example, a paper surface. The light might be modulated by the spot absorbing light, reflecting light, scattering the light, causing fluorescence, or otherwise affecting the light. This change in a property of the light will be indicative of a characteristic of a signal generating compound in the spot. The light can be sensed with any number of devices, such as charge coupled devices, charge-injection devices, such as charge coupled devices, charge-injection devices, or photodiodes. These devices operate according to photoelectric effects by producing an electrical response in proportion to the amount of light impinging on the device. This analog signal can be sampled to determine the amount of light impinging upon the device. These analog values can be converted into digital value with an A/D (analog to digital) converter so that the data may be analyzed by a computer. The actual value of the signal can be measured or the amount of time required to reach a specified signal can be measured to determine the amount of light impinging on the device.
Outside the chemical field in the area of image processing, a recent development is the use of the optical dynamic RAM (random access memory) for image processing. A dynamic RAM has a series of memory cells with each cell acting like a storage capacitor. The storage capacitor will either have a voltage stored on it, voltages above or below a reference level indicated as a digital 1 or 0 value, respectively. Because leakage current causes the voltage on a capacitor to decay, each memory cell is periodically refreshed in a dynamic RAM to maintain the integrity of the data. If the surface of the RAM is exposed to light, this light will cause a photocurrent which will produce an additional leakage of current. The amount of light impinging on the RAM can be determined by measuring the amount of time for each memory cell to decay from a logical 1 to a logical 0.